1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor.
2. Description of the Related Art
A multilayer ceramic capacitor is composed of an element body having the configuration that a plurality of dielectric layers and internal electrode layers are alternately stacked and a pair of external terminal electrodes formed on both end portions of the element body. The multilayer ceramic capacitor is produced by producing a pre-firing element body by alternately stacking pre-firing dielectric layers and pre-firing internal electrode layers exactly by necessary numbers first, then, after firing the same, forming a pair of external terminal electrodes on both end portions of the fired element body.
When producing a multilayer ceramic capacitor, since the pre-firing dielectric layers and the pre-firing internal electrode layers are fired at a time, a conductive material included in the pre-firing internal electrode layers is demanded to have a higher melting point than a sintering temperature of dielectric material powder included in the pre-firing dielectric layers, not to react with the dielectric material powder, and not to be dispersed in the fired dielectric layers, etc.
In recent years, to respond to the demands, as a conductive material included in the pre-firing internal electrode layers, instead of conventionally used Pt, Pd and other precious metals, an Ag—Pd alloy is used, or those using Ni, which can be fired in a reducing atmosphere, and other inexpensive base metals by giving reduction-resistance to the dielectric material have been developed.
The case of using Ni as a conductive material included in the pre-firing internal electrode layers will be explained as an example. Ni has a lower melting point comparing with that of dielectric material powder included in the pre-firing dielectric layer. Therefore, when pre-firing dielectric layers and pre-firing internal electrode layers including Ni as a conductive material are fired at a time, due to a difference of sintering start temperatures of the dielectric material powder and Ni, Ni internal electrode tends to become thick to be eventually broken as sintering of the dielectric material powder proceeds. Thus, to suppress this kind of breaking due to firing and to suppress sintering, there is proposed a technique of adding an additive dielectric material as a sintering retarder to an internal electrode layer paste for forming the internal electrode layers (refer to the patent articles 1 to 5). The additive dielectric material has a property of being dispersed from the internal electrode layer side to the internal dielectric layer side at the time of firing pre-firing internal dielectric layers and pre-firing internal electrode layers at a time.
In recent years, as a result that a variety of electronic apparatuses became compact, a multilayer ceramic capacitor installed inside the electronic apparatuses has been demanded to realize a compact body with a larger capacity, a low price and high reliability. To respond to the demands, a fired internal electrode layer, fired internal dielectric layer arranged between mutually facing fired internal electrode layers have been made thinner. Specifically, a thickness after firing per one fired internal dielectric layer has become as thin as 1 μm or so and, along therewith, a thickness before firing per one pre-firing internal dielectric layer has also become thinner.
As the pre-firing internal dielectric layer becomes thinner, a content of a dielectric material per one dielectric layer for forming it decreases.
For example, the case of preparing an internal electrode layer paste obtained by adding an additive dielectric material at a predetermined weight ratio to Ni as a conductive material and forming by applying the paste to be a predetermined thickness to a plurality of pre-firing internal dielectric layers, wherein the pre-firing thickness is gradually made thinner, will be considered. At this time, the weight ratio of a content of the additive dielectric material in the internal electrode layer with respect to a content of the dielectric material in the pre-firing internal dielectric layer (a content of the additive dielectric material in the internal electrode layer/a content of the dielectric material in the pre-firing internal dielectric layer) gradually increases as the thickness of the pre-firing internal dielectric layer applied with the internal electrode layer paste becomes thinner. It is because a content of the dielectric material in the pre-firing internal dielectric layer decreases as the thickness of the pre-firing internal dielectric layer becomes thinner, so that a denominator of a formula of the above weight ratio becomes smaller, consequently, a value of the weight ratio becomes larger.
When considering this from the pre-firing internal dielectric layer side, it means that the thinner the thickness becomes, the larger an amount of the additive dielectric material to be dispersed from the internal electrode layer side relatively becomes. Namely, a relative dispersal amount from the internal electrode layer side to the internal dielectric layer side increases.
Also, as the pre-firing internal dielectric layer becomes thinner as above, the pre-firing internal electrode layer is also demanded to be thinner, however, to make the pre-firing internal electrode layer thinner, the additive dielectric material as well as a conductive material, such as Ni, in the internal electrode layer paste for forming the same are demanded to be finer.
However, when the additive dielectric material to be dispersed from the internal electrode layer side to the internal dielectric layer side at the time of firing is made finer, grain growth of dielectric particles composing the internal dielectric layer may be accelerated to influence the fine structure of the internal dielectric layer in some cases. As explained above, the influence is furthermore enhanced when the dispersal amount of the additive dielectric material from the internal electrode layer side to the internal dielectric layer side becomes larger. The influence on the fine structure can be ignored when a thickness of the fired internal dielectric layer is made to be 2.0 μm or more, however, the influence on the fine structure tends to become large when the thickness of the fired internal dielectric layer is made thin as less than 2.0 μm. Along with the influence on the fine structure as such, it is liable that various characteristics, such as a bias characteristic and reliability, of a multilayer ceramic capacitor to be obtained are deteriorated.
To solve the disadvantages, the patent article 6 proposes a technique of adjusting an additive composition for an internal electrode layer paste and adjusting a ratio of an average particle diameter of dielectric particles contacting the internal electrode layer after firing and that of not contacting dielectric particles, concentration ratio of additive components and a core-shell ratio. According to the technique described in the patent article 6, a dielectric layer can be made thinner without deteriorating a temperature characteristic, i and lifetime. However, bias characteristics were not sufficiently improved in the technique described in the patent article 6, so that a problem to be solved still remained.
The patent article 7 discloses a multilayer ceramic capacitor wherein an average particle diameter of dielectric particles near an external electrode is the same as or smaller than an average particle diameter of dielectric particles in an effective region.
However, the technique described in the patent article 7 is for a purpose of preventing cracks at the time of sintering the external electrode, and an improvement of the bias characteristics cannot be expected.                Patent Article 1: The Japanese Unexamined Patent Publication No. 5-62855        Patent Article 2: The Japanese Unexamined Patent Publication No. 2000-277369        Patent Article 3: The Japanese Unexamined Patent Publication No. 2001-307939        Patent Article 4: The Japanese Unexamined Patent Publication No. 2003-77761        Patent Article 5: The Japanese Unexamined Patent Publication No. 2003-100544        Patent Article 6: The Japanese Unexamined Patent Publication No. 2003-124049        Patent Article 7: The Japanese Unexamined Patent Publication No. 2003-133164        